The present invention is directed to the use of polyurethane based hot melt adhesives for bonding the paper which wraps the filter element of a cigarette and to cigarette filter elements bonded therewith.
A standard filter tipped cigarette is produced by three different operations: (1) filter or plug manufacture, (2) rod manufacture and (3) tipping.
The cigarette rod consists of a stream of tobacco wrapped in a paper tube, the paper tube being glued along one longitudinal edge with a suitable adhesive. This operation takes place at speeds varying from 2000-12000 cigarettes per minute. The weight of the cigarette rod, diameter, and length are all closely monitored for both cost control and tax reasons.
The filter making operation is similar in many respects to the rod manufacture, but the materials used are different. Tobacco smoke filters are most commonly manufactured from a crimped textile tow of cellulose acetate fibers. The tow is then warmed and rolled to produce a material about 9" wide, and containing an even distribution of fibres. The tow is "bloomed" in a known manner to separate the fibers, and a high boiling solvent, commonly called a "plasticizer", is applied by spraying, wicking, or other suitable means. The treated tow is then pulled into a cylindrical form and wrapped with paper. During an interval of time, which can be accelerated by heating, the plasticizer first partially dissolves the surface of the fibers causing them to become sticky and to bond together at points of contact with each other. The plasticizer then migrates into the fiber leaving the surface dry, but the fibers still remain bonded.
The bloomed tow is then formed into a tube, and wrapped in paper or plugwrap, the paper being glued along one longitudinal edge. Because of the nature of the tow, there is a high degree of radial stress upon the glued bond, so any adhesive used, must be able to stick almost instantly, and hold indefinitely. This has led to the use of hot melt type adhesives, which by formulation are fast setting, and allow very high machine speeds to be used.
However, the plasticizers used in the blooming operation attack hot melts, acting as a solvent for the hot melt. This attack on the hot melt reduces the shelf life of the finished plug to a matter of weeks. In practice, this means that cigarette companies are unable to produce plugs for stock, and so work very much on a "made today-use tomorrow" basis. This lack of stock can lead to (1) poor machine maintenance of the plug makers, leading to shorter machine life; (2) insufficient plugs for producing cigarettes; and/or (3) wrong tips or specification plugs, again with machine downtime.
Furthermore, use of plugs which have been stored for any appreciable length of time could cause excessive machine downtime caused by blockages in the production process since the plugs are fed along narrow pipes and if the plug comes apart, the pipe is blocked. Even if the cigarettes can be assembled using these plugs, the results will be poor quality cigarettes.